Method for strengthening boron filaments for use in forming filamentary composites

ABSTRACT

Boron filaments are laid out linearly, either vertically or horizontally, and held at a predetermined temperature for a predetermined period of time. The filaments are also preferably exposed to a gas or gaseous mixture including nitrogen. The heating in the manner described stress-relieves the filaments, while the exposure to the gas heals defects in the surface thereof, substantially improving the tensile strength and variability in strength of the filament.

United States Patent (1 1 [111 3,917,783 Waterman et al. Nov. 4, 1975 METHOD FOR STRENGTHENING BORON Primary Examiner-Donald J. Arnold FILAMENTS FOR USE IN FORMING Assistant ExaminerJohn Parrish FILAMENT COMPOSITES Attorney, Agent, or Firm-Edward A. Sokolski [75] Inventors: Dwight Ray Waterman, San Pedro;

Gary G. Brown, Diamond Bar; George .1. Mills, Santa Ana, all of Calif.

[73] Assignee: Northrop Corporation, Los Angeles, ABSTRACT Calif.

[22] Filed: 1973 Boron filaments are laid out linearly, either vertically [2l] Appl. No.: 390,596 or horizontally, and held at a predetermined temperature for a predetermined period of time. The filaments are also preferably exposed to a gas or gaseous mix- [52] 264/85 264/DIG' gtg ture including nitrogen. The heating in the manner described stress-relieves the filaments, while the expog E h l: 522 3 4 sure to the gas heals defects in the surface thereof, 1 l m substantially improving the tensile strength and vari- 264/85, 230, 231, DIG. 28, 235, 346, 238;

106/55 abillty in strength of the filament.

[56] References Cited UNITED STATE PATENT 3 Claims, 3 Drawing Figures 2,952,033 9/1960 Goodwin 264/DIG. 28 3,668.059 6/!972 Economy et al l6l/l69 3,775,530 11/1973 Lawrence et al. l. 264/60 l2 MW.

I/ III! METHOD FOR STRENGTHENING BORON FILAMENTS FOR USE IN FORMING FILAMENTARY COMPOSITES This invention relates to a method for improving the tensile strength of boron filaments while also reducing the dispersions of strengths of boron filaments, and more particularly to such a method suitable for use in strengthening filaments for use in forming structural composites.

High strength filamentary reinforced structural composites can be formed by incorporating boron filaments in a plastic resin or metal matrix material. it has been found that a substantial improvement in the composite properties can be achieved by increasing the tensile strength and decreasing the dispersion in strength of the boron filaments utilized by a simple yet highly effective technique. The present invention is directed to this technique which basically involves stress-relieving boron filaments by maintaining the filaments at a predetermined temperature for a predetennined period of time (depending on the temperature), while maintaining the filament in a substantially linear condition, and in a preferred embodiment by also simultaneously subjecting the surface of the filament to a gas such as nitrogen, or a mixture including nitrogen, to heal any surface discontinuities such as imperfections or flaws which may appear on such surface.

It is therefore an object of this invention to improve the strength of boron filamentary composites.

lt is another object of this invention to provide means for strengthening boron filaments to provide a substantial improvement in the structural characteristics of filamentary composite formed with such filaments.

Other objects of this invention will become apparent as the description proceeds in connection with the accompanying drawings, of which:

FIG. 1 illustrates the implementation of a first embodiment of the technique of the invention; and

FlGS. 2 and 2A illustrate the implementation of a second embodiment of the technique of the invention.

Briefly described, the technique in the invention is practiced by maintaining the boron filaments at a predetermined temperature for a predetermined period of time, the filaments being held in an essentially linear condition during this period of time. During this time, preferably a gas comprising nitrogen as the main ingredient thereof is scrubbed over the surface thereof to heal any surface discontinuities on or near the surface.

Referring now to FIG. 1, a boron filament ll of any practical length is suspended from support 12 in heating container 14, the filament being maintained in a linear condition by means of weight 15 attached to one end thereof. Heat is applied to the filament by means of strip heater 19. The filament may be of the type commercially available from the Systems Division of AVCO Corporation, Lowell, Massachusetts, or from the Hamilton Standard Division of United Aircraft Corporation. Such filaments are fabricated by the vapor deposition of boron on a tungsten substrate as described, for example, in an article by Hoffman and Ditt- .mer entitled Boron Composites: Status in the U.S.A.,

which appeared in the June 1973 edition of 1N- TERAVlA magazine.

Referring now to FIGS. 2 and 2A, a second means for implementing the technique of the invention is schematically illustrated. Filament ll is supported in chamber 14 in a linear condition between spools l7 and 18. Strip heater 19 is utilized to provide heat to the filament. Chamber 14 is airtight and has an outlet 20 which may be used for purging or evacuating the chamber and an inlet 22 which may be used for supplying a gas such as nitrogen to the chamber. The following are examples of the technique of the invention:

EXAMPLE 1 A boron filament 11 was hung as indicated in FIG. 1 for a period of one month at room temperature. The average tensile strength of 3 inches length samples after such hanging increased from 462.5 ksi with a coefficient of variation of 15.6% to 558.3 ksi/ 14.7%.

EXAMPLE ll A boron filament 11 was placed in chamber 14 as indicated in FIGS. 2 and 2A. The chamber was purged of air by means of outlet 20 and the chamber was filled with commercial purity nitrogen through inlet 22. The temperature of the filament was raised to 250F by means of heater 19. The filament was maintained at this temperature for 20 minutes while being subjected to the nitrogen environment. The average tensile strength of 3 inches length samples taken from the filament increased from 462.5 ksi with a coefficient of variation of 15.6% prior to treatment to 560.3 ksi/9.6%.

EXAMPLE Ill The filament 11 was placed in chamber 14 as indicated in FIGS. 2 and 2A and the chamber evacuated to provide a vacuum environment. The filament was heated to 200F and maintained at this temperature in the vacuum environment for a period of four hours. The average tensile strength of the 3 inches long filament samples increased to 502.1 ksi/7.3% from a pretreatment tensile strength of 462.5 ksi/ 15.6%.

EXAMPLE [V The filament was placed in chamber 14 as indicated in FIGS. 2 and 2A and the chamber filled with pure nitrogen. The filament was heated to 300F by means of the heater and maintained at this temperature in the nitrogen environment for a period of 5 minutes. The average strength of the 3 inches length filament was found to have increased to 592.3 ksi/8.9% from 462.5 ksi/ 15.6% prior to treatment.

EXAMPLE V The filament was placed in chamber 14 as indicated in FIG. 2, and heated to 200F by means of the heater. The chamber was filled with air and the filament maintained at the 200F temperature for 1% hours in such air environment. The chamber was then evacuated and the filament kept in a vacuum environment at the 200F temperature for an additional 1% hour. The tensile strength of the filament was found to have increased to 547 ksi/9.0% from an initial tensile strength of 462.5 ksi/ 15.6%.

The tests of tensile strength in all of the above examples were made by applying tensioning force to the filaments along their longitudinal axes.

It is believed that the maintenance of the filament at a predetermined temperature for a predetermined period of time while it is maintained in a linear state stressrelieves the filament and in this manner improves its strength, while the scrubbing of the surface of the filament with a gas such as nitrogen tends to heal cracks and other surface discontinuities on the surface of the filament and thus further improves its tensile strength. The filaments so strengthened, as already noted, can be utilized to form a filamentary composite having improved structural characteristics, the filaments being impregnated in a resin or metal binder material by techniques well known in the art.

While the technique of the invention has been described and illustrated in detail, it is to be clearly understood that this is intended by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the invention being limited only by the terms of the following claims.

We claim:

1. A method for treating a boron filament to improve the tensile strength thereof comprising the steps of:

placing at least a portion of such filament under tension in a substantially linear configuration, and

maintaining the filament at a temperature of 200-350F for a period of time sufficient to increase the tensile strength of said filament, said filament being maintained in a chamber filled with substantially pure nitrogen during said period to heal defects on the surface thereof but not so as to nitride said surface.

2. The method of claim 1 wherein said filament is brought to a temperature of 300 to 350F and maintained at this temperature for approximately 5 minutes.

3. The method of claim 2 wherein the filament is maintained at said temperature for 20 minutes while being subjected to the nitrogen environment. 

1. A METHOD FOR TREATING A BOROM FILAMENT TO IMPROVE THE TENSILE STRENGTH THEREOF COMPRISING THE STEPS OF: PLACING AT LEAST A PORTION OF SUCH FILAMENT UNDER TENSION IN A SUBSTANTIALLY LINEAR CONFIGURATION, AND MAINTAINING THE FILAMENT AT A TEMPERATURE OF 200*-350*F FOR A PERIOD OF TIME SUFFICIENT TO INCREASE THE TENSILE STRENGTH OF SAID FILAMENT, SAID FILAMENT BEING MAINTAINED IN A CHAMBER FILLED WITH SUBSTANTIALY PURE NITROGEN DURING
 2. The method of claim 1 wherein said filament is brought to a temperature of 300* to 350*F and maintained at this temperature for approximately 5 minutes.
 3. The method of claim 2 wherein the filament is maintained at said temperature for 20 minutes while being subjected to the nitrogen environment. 